Polyacrylonitrile fiber has characteristics such as bulkiness, texture, and clearness in dying similar to wool, and is used in a variety of uses. Polyacrylonitrile fiber is also generally used as precursor of carbon fiber.
Polyacrylonitrile fiber is generally produced by wet or dry-jet-wet spinning of spinning dope in which an acrylonitrile copolymer is dissolved in an organic or inorganic solvent for transformation of the spinning dope into fiber followed by stretching, washing, and drying for densification of the thus formed fiber.
As a solvent for the spinning dope, an amide solvent such as dimethylformamide or dimethylacetamide, dimethylsulfoxide, or the like is widely used.
However, there is a problem such that storage stability of spinning dope is not good when an amide solvent is used as a solvent for the spinning dope.
As a counter measure for such a problem, for example, there is disclosed a technology in Patent Document 1 in which an acrylonitrile copolymer is dissolved in an amide solvent such as dimethylacetamide in which the content of dimethylamine is not more than a certain amount and hence stability of a thus obtained solution (spinning dope) is improved. By such a technology, gelation and coloring are suppressed in long time storage. However, as for the aforementioned solution, there is a case where stability of the solution under high temperature (thermal stability) is insufficient because, for example, hydrolysis of dimethylacetamide occurs with time under a high temperature and a cyclization reaction of the polymer is allowed to proceed by dimethylamine generated through the hydrolysis.
In Patent Document 2, it is disclosed that polyacrylonitrile fiber is obtained from a solution (spinning dope) in which an acrylonitrile copolymer having not less than a fixed amount of sulfate group and/or sulfonate group at the end of the polymer is dissolved in dimethylacetamide. The polyacrylonitrile fiber thus obtained is excellent in denseness and suitable for production of carbon fiber. However, there is a case where thermal stability of the solution is insufficient, the same as in the aforementioned case.
In Patent Document 3, it is disclosed that viscosity change of a solution (spinning dope) is suppressed by dissolution of an acrylonitrile copolymer having not less than a fixed amount of sulfate group and/or sulfonate group into a solvent such as dimethylsulfoxide which does not have an amide bond. Such a solution is preferable from the viewpoint of stability. However, there is a problem such that polyacrylonitrile fiber obtained by use of the aforementioned solution is inferior in denseness to that obtained by use of an amide solvent.